An aqueous solution of hypochlorous acid, i.e. a water containing hypochlorous acid whose pH is adjusted to provide a weak acidity (a water containing HOCl), is reasonably inexpensive and has a great sterilizing power, while being of a high safety to a human body. Therefore, such aqueous hypochlorous acid solution finds enormous uses in various fields, including a typical exemplary use as a sterile water for cleaning purposes in medical facilities, food industries, and so forth. Various sorts of sterile water production devices capable of continuously producing a great amount of the sterile water of this kind have been manufactured and made available on markets.
Among the foregoing sterile water production devices, there is, for instance, a mixing type of devices operable for adding hydrochloric acid solution and aqueous hypochlorite solution (e.g. aqueous solution of sodium hypochlorite) to a raw water (e.g. tap water or well water) and intermixing those three liquids together, so as to continuously produce a great amount of sterile water containing hypochlorous acid therein. An example of this particular mixing-type sterile water production device is taught in the Japanese Laid-Open Patent Publication No. 2003-401047.
In this context, as understandable from chemical equation in the “Chem 1” below, a hypochlorite present in the aqueous hypochlorite solution becomes decomposed and inactive with the passage of time, and further, deactivation of that hypochlorite becomes rapid in the case that the aqueous hypochlorite solution is stored at a high ambient temperature. Consequently, if the aqueous hypochlorite solution that has been unused for a long period of time or stored at a high ambient temperature is intermixed and reacted with hydrochloric acid solution and raw water as stated above to produce a sterile water, a concentration of hypochlorous acid in the resulting sterile water is actually smaller and less sufficient than expected, because the concentration of hypochlorous acid in the sterile water is reduced to a level equal to an amount of the decomposed and/or deactivated portion of hypochlorite content in this deteriorated kind of aqueous hypochlorite solution.2NaOCl→O2↑+2NaCl  [Chem 1]
The concentration of residual chlorine (or hyphochlorous acid) in sterile water is an important factor for retaining a sterilizing power of the sterile water, because the sterilizing power is dependent upon a concentration of the residual chlorine as well as upon a pH of the residual chlorine. It is therefore necessary that, according to a degree of deactivation of hypochlorite, an appropriate additional amount of aqueous hypochlorite solution should be injected to insure that a residual chlorine concentration in sterile water being produced is maintained to a preset degree.
For that purpose, the above-described type of sterile water production device requires incorporating a residual chlorine densitometer to continuously determine a residual chlorine concentration in a sterile water being produced by that device, and also requires a control system for controlling and adjusting the residual chlorine concentration, such that, upon detecting a decreased concentration of residual chlorine in the sterile water, a correspondingly increased amount of aqueous hypochlorite solution is injected into the sterile water production device, to thereby increase the residual chlorine concentration to the preset degree.
However, the residual chlorine densitometer is an extraordinarily expensive meter, and therefore, incorporation of such densitometer in the foregoing type of sterile water production device results in providing an undesirably high-priced sterile water production device. In view of such cost problem, some inepensive sterile water production devices have been manufactured and made available on market, which each employs a pH meter for measuring pH of sterile water being produced, instead of the residual chlorine desitometer, and operates to detect and adjust changes of pH in the sterile water to a normal pH level, thereby indirectly controlling a residual chlorine concentration in the sterile water.
The foregoing control of residual chlorine concentration via pH meter is based on the following chemical adjustment procedures: When aqueous hypochlorite solution containing an insufficient amount of hypochlorite due to the foregoing deactivation reasons is injected in the presence of hydrochloric acid, a certain amount of the hydrochloric acid naturally remains intact as a surplus hydrochloric acid in a sterile water produced, since the amount of hypochlorite is insufficient for full reaction with that hydrochloric acid, thus resulting in the sterile water produced being in a low pH level, and, in order to adjust and restore such low pH level to a preset pH, additional injection of appropriate amount of aqueous hypochlorite solution effectively causes the surplus hydrochloric acid to be neutralized therewith and absent, whereby the pH is increased and restored to a preset pH and therefore a desired concentration of residual chlorine is contained in a resulting sterile water.
Theoretically, the above-described pH control method is feasible on the understanding that the aqueous hypochlorite solution contains very few residual alkaline impurities therein. But, in the case that the residual alkaline impurities are contained at an appreciable rate (0.3 to 2.4%) in the aqueous hypochlorite solution, it must be noted that a substantive rate of injected hydrochloric acid reacts with such appreciable rate of residual alkaline impurities, and thus, the rate of the hydrochloric acid in a sterile water produced decreases accordingly, which means that the residual alkaline impurities serve to adjust and restore a decreased pH to a preset normal pH level. Hence, in spite of a pH meter indicating such preset pH for the sterile water produced, an actual amount of the hypochlorous acid present in that sterile water, which is yielded by reaction between the aqueous hypochlorite solution and the hydrochloric acid solution, is still insufficient to attain a desired concentration of residual chlorine in the sterile water. Without noticing such fact, to merely continue injection of the aqueous hypochlorite solution and monitoring pH of produced sterile water in the foregoing manner will eventually result in a progressive reduction of residual chlorine concentration in the serial water, as a consequence of which, a sterilizing power of finally produced sterile water decreases considerably.                Cited Patent Literature 1: the Japanese Laid-Open Patent Publication No. 2003-401047        